Conventional microporous membranes made from polymers (e.g. polycarbonate, polysulfones and polyvinylidene fluoride) are used to filter a wide variety of liquids and gases. Many polymeric membranes by themselves, however, have relatively poor mechanical properties. These membranes are fairly weak and may not maintain their integrity under normal handling. They will stretch when pulled and will shrink upon heating. They are also sensitive to folding and may crack at pleats that develop when folded. Furthermore, the mechanical assembly of unsupported polymeric membranes in devices frequently requires a thermoplastic component in order to seal the membrane at a seam or cap.
Where polymeric membranes are, however, structurally supported, the membranes typically are formed with an external, disposable mesh. This adds cost to the entire process and requires the additional step of removing the used membrane from the disposable mesh.
Another type of support is one in which a non-disposable, external support such as a polypropylene scrim or grid is laminated to the membrane.
Polymeric membranes are also made with internal supporting systems. In one membrane of this type, a nonwoven grid is incorporated into the polymer internally as the polymeric membrane is made. The grid and membrane are made of different materials. Nevertheless, polymeric membranes with internal supporting systems made of different materials can overcome only some of the problems associated with unsupported membranes. A serious problem encountered during production and use of supported membranes is that, under certain conditions, the external or internal grid can delaminate from the membrane. Because the materials of the grid and surrounding membrane are different, the thermal properties of the unit as a whole is poor because the grid and membrane have different rates of thermal expansion and contraction.